1. Field of the Invention
This invention relates to a method and apparatus for processing digital signals over a wide frequency range, such as digital audio signals, a method for transmitting digital signals, and a recording medium. More particularly, it relates to a method and apparatus for processing digital signals, a method for transmitting digital signals, and a recording medium, in which the frequency spectrum is divided into plural bands for a lower range one of which and for a higher range one of which straight PCM and high-efficiency encoding are applied, respectively.
2. Description of Related Art
Recently, recording/reproduction of digital audio signals is employed extensively. In particular, standards for a compact disc (CD) or a digital audio tape recorder (DAT) have become widespread as typical standards for recording/reproduction of digital audio signals.
As for these standards for digital audio signals, since the sampling frequency for the compact disc is prescribed at 44.1 kHz, the maximum reproducing frequency is 22.05 kHz. On the other hand, since the sampling frequency for a digital audio tape recorder (DAT) is prescribed at 32 kHz, 44.1 kHz and 48 kHz, the maximum reproducing frequency is 16 kHz, 22.05 kHz and 24 kHz, respectively.
Meanwhile, in the case of a sound source of folk music, such as gamran or yodel voice, there are contained significant amounts of frequency components exceeding the above-mentioned maximum reproducing frequency. Thus the above-mentioned sampling frequency of 44.1 kHz or 48 kHz is no longer sufficient insofar as the above-given folk music is concerned.
On the other hand, a digital/analog (D/A) converter has recently been improved in precision and has become able to handle signals of weaker strength. Thus the dynamic range on the order of 98 dB, obtained with the number of quantization bits for the CD, has become no longer sufficient.
For combatting this problem, it may be contemplated to record signals improved in sound quality by increasing the sampling frequency or the number of quantization bits, that is the signals of broader bands or broader dynamic ranges, on a disc (recording medium) having the same size as the conventional CD.
From the technical viewpoint, the signals improved in sound quality by increasing the sampling frequency or the number of quantization bits can be recorded on the disc (recording medium) of the conventional size (recording capacity) by reducing the track pitch of the disc or the laser light wavelength of the optical pickup. However, if the device configuration is modified for increasing the recording capacity, it becomes impossible to maintain interchangeability with the conventional recording media, while confusion is produced in the software market.
For recording/reproducing the sound having the sampling frequency exceeding the maximum reproducing frequency without modifying the device or formats of the standards for conventional digital audio signals, that is without impairing compatibility with the conventional standards, it may be contemplated to provide such a signal processing method, recording/reproducing device or a recording medium in which the reproducing band for the CD is encoded with straight PCM and higher bands are encoded using high efficiency encoding, thereby recording the signals n the usual recording frequency range for the compact disc. With such method or device, the usual PCM word is divided, for recording the signals in the usual recording range for the compact disc, so that the usual reproducing range is encoded with straight PCM and higher bands are encoded using high efficiency encoding. The encoded signals are recorded on the compact disc.
In this case, the PCM word employed for the conventional reproducing range becomes naturally shorter in length than the conventional PCM word, so that the quantization noise is produced, thereby deteriorating the sound quality.
In addition, when the usual reproducing range and the higher range are encoded with straight PCM and with high-efficiency coding, respectively, and the encoded signals are recorded on the compact disc, there is raised a problem as to how the usual PCM word is to be divided. That is, if the frequency spectrum of the digital audio signals is divided as described above, some bands have allowance in recording capacity, while others do not, in dependence upon signals, such that, in the bands devoid of allowance in recording capacity, the quantization noise becomes outstanding because the number of quantization bits or the sub-word length is diminished for compensating for scarcity in the recording capacity.